BASALTIC AND IGNEOUS ROCKS. 641 
of basalt or basaltic lava have flowed throughout the land. We have 
shown, in another place, that fissure eruptions are common in all 
volcanic regions, (at least in recent periods,) and the same fact is 
sustained by a survey of Oregon. 
The three peaks of the Cascade Range, Rainier, St. Helen’s and 
Mount Hood, were so far examined by the surveying parties of the 
Expedition, as to determine that the rocks became more cellular and 
lava-like as the mountains were approached. Near Mount Rainier, 
Dr. Pickering found trachytes abundant; over the foot of St. Helen’s, 
the rocks were cellular basaltic lavas; along the Cascades of the 
Columbia, and on the John Day’s and Chutes rivers, which are pro- 
perly at the foot of Mount Hood, there were similar cellular lavas, as 
ascertained by Mr. Drayton. It also appears that remains of a crater 
may be distinguished in the summit of Mount Rainier. These cones 
have similar features, as already described, and slope at an angle of 
about thirty degrees. Mount Saint Helen’s is quite regularly conical. 
Shasty Volcanic Region.—The Shasty Peak first came in view on 
the expedition to California, fifty miles to the north of it, where a 
wide prairie opened before us, and stretched away to the foot of the 
mountains. ‘Travelling on six miles, a low ledge of the tertiary sand- 
stone extended across the prairie from east to west, dipping fifteen 
degrees to the northward. Six miles farther, we passed a heap of vol- 
canic rocks, consisting of large masses of grayish and reddish porphy- 
ritic lava, one to ten cubic feet in size, lying together in a disorderly 
pile. As we continued on, these heaps of lava blocks became frequent, 
and the plain was found covered with rounded and conical hillocks 
from twenty to two hundred feet in height, but averaging sixty feet. 
Some few had table summits. Although mostly covered with soil, the 
black rocks outcrop at top, and lie scattered over the surface; and not- 
withstanding they are covered with a red or brownish-red earth from 
decomposition, this earth is scarcely at all mingled with the alluvium 
of the plain. On the contrary, the prairie soil was of sandstone origin, 
and proved that this hellock prairie had been levelled under water 
since the volcanic rocks were thrown up. 
Five miles from the first appearance of these lava hillocks, they are 
so crowded together that the plains almost disappear between their 
approaching declivities. Nearing the mountains, the country becomes 
a region of rough rounded hills, which increase in extent, and rise 
into high ridges lying at the base of the Shasty Peak. Some of these 
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